A convenient method of preparing "overexchanged" FeZSM-5 catalysts is described that minimizes oxidation of Fe2+ to Fe3+ during base-exchange and inhibits the formation of Bronsted sites. These catalysts are more active than comparable CuZSM-5 preparations and are much more durable. They are not poisoned by up to 20% H2O and/or 150 ppm of SO2. A 2500-h durability test, carried out at 500 degrees C, is described. It showed minimal loss of activity in a simulated exhaust gas stream. Moreover, 24-h in situ excursions to higher temperatures (up to 800 degrees C) caused no loss in activity on return to 500 degrees C. These findings are contrasted with those obtained with CuZSM-5. Examination of the catalyst at the end of the test showed minimal loss of pore volume and only T-d coordinated Al-27 could be detected. The sum of Fe + Na (residual) was equal to the Al. (1)HNMR showed no evidence of Bronsted sites suggesting that (FeOH)+ was exchanged with Na+ with the corresponding H+ being removed by the anion of Fe-salt used for exchange, viz., C2O4-2. Supplemental tests showed that the Fe2+ base-exchanged cations could be readily oxidized to Fe3+ by O-2 and reduced back to Fe2+ with H-2, but could not be further reduced at temperatures as high as 950 degrees C. The results suggest that dealumination and breakdown of the lattice result from the presence of Bronsted sites.